Monoclinic hydroxyapatite was synthesized by a wet method followed by heating in air at 1473 K for 1 h. The product is carbonate-free and has a Ca/P ratio of 1.65, which is slightly lower than the theoretical value (1.67) of hydroxyapatite. The crystal structure was investigated by Rietveld refinement using X-ray powder diffraction data. The structure has monoclinic symmetry with the space groupP21/b(No. 14),Z=4,Dx=3.14 g cm-3, and the lattice constants area=9.426(3) Å,b=18.856(5) Å,c=6.887(1) Å, andγ=119.97(1)°. The hydroxyl ions have an ordered arrangement along the hydroxyl columns. The hydroxyapatite prepared in the present study is 98% monoclinic.
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